This invention relates to a circuit arrangement for operating a lamp, provided with
input terminals for connection to a supply voltage source which delivers a low-frequency AC voltage, PA1 rectifying means coupled to the input terminals for rectifying the low-frequency AC voltage and provided with output terminals, PA1 a first branch comprising a series arrangement of a breakdown element and first capacitive means and interconnecting the output terminals, and PA1 a second branch shunting the first capacitive means for discharging said first capacitive means. PA1 further capacitive means across which a DC voltage is present during lamp operation, with which voltage the lamp is supplied, PA1 a DC--DC converter coupled to the output terminals and the further capacitive means, and PA1 a third branch comprising a series arrangement of diode means and the further capacitive means which shunt the first capacitive means.
Such a circuit arrangement is known from German patent document DE-OS 2100931. The second branch in the known circuit arrangement is formed by a resistor. If the low-frequency AC voltage is supplied by an AC mains which delivers the same low-frequency AC voltage to a (usually large) number of users, transients may arise due to various causes. Transients are here understood to be voltage peaks of comparatively high amplitude (often of the order of 1000 volts) which are superimposed on the low-frequency AC voltage during a comparatively short time. If the circuit arrangement is not provided with a transient suppressor, the occurrence of such transients may damage the components from which the circuit arrangement is built up. In the known circuit arrangement, the first branch and the second branch together form a transient suppressor. When a transient occurs, the breakdown voltage of the breakdown element is exceeded so that this element becomes conductive and charges the first capacitive means. Since the resistance of the breakdown element in the conductive state is comparatively low, the current with which the first capacitive means are charged is comparatively great, so that the transient is strongly suppressed. When the voltage across the first capacitive means has risen to a value equal to the instantaneous value of the voltage between the output terminals, the current through the breakdown element becomes zero and the breakdown element accordingly becomes non-conductive. The first capacitive means are now discharged through the resistor by which it is shunted. The resistor in the known circuit arrangement is a resistor with a positive temperature coefficient. It is achieved thereby that the resistor has a comparatively high value very quickly after breakdown of the breakdown element, so that the current flowing through the breakdown element is largely used for charging the first capacitive means. If the resistance value of the resistor were too low, the first capacitive means would not be charged sufficiently for rendering the breakdown element non-conductive. The comparatively high resistance value of the resistor at least immediately after the breakdown element has become non-conductive means that the discharge of the first capacitive means, after the suppression of a transient, takes comparatively long. If a next transient occurs before the first capacitive means have been fully discharged, the capacity of the transient suppressor for suppressing the transient may have been considerably reduced by a comparatively high instantaneous value of the voltage across the first capacitive means. Components of the circuit arrangement may thus be damaged when two transients occur immediately after one another. A similar problem arises when a transient of comparatively high energy content occurs. It is suggested in DE-OS 2100931 to counteract this disadvantage in that further transient suppressors of similar construction are included in the circuit arrangement parallel to the first transient suppressor, the breakdown voltage of the breakdown element of each further transient suppressor being chosen to be higher than that of the breakdown element in the preceding transient suppressor. Such a solution, however, requires comparatively many and comparatively expensive components so that the circuit arrangement is comparatively expensive.